A passive optical network typically consists of a transmitter, feeder fiber, a remote branching device, and an optical network unit (ONU) for each subscriber or group of subscribers. In a conventional TDM power splitting passive optical network, the transmitter emits an aggregated signal on a single wavelength, while the branching device splits the aggregated signal into a plurality of identical signals, each such signal being distributed to a corresponding ONU configured to receive at the aggregate bit-rate.
In a wavelength division multiplexed (WDM) passive optical network, each transmitted wavelength carries data for a single ONU and is modulated at the baseband bit rate. The branching device, which may comprise, for example, a wavelength grating router, establishes a virtual point-to-point link between the transmitter and the ONU. A multifrequency optical source such as a multifrequency laser or a WDM laser array is customarily used to generate the comb of wavelengths required for the WDM network.
Since data for each wavelength channel is encoded directly at the multifrequency optical source, for example, by turning on and off individual elements of a multifrequency laser array, one WDM multifrequency laser is required for each N subscribers or optical network units, where N is the number of wavelengths emitted by the laser. Due to the high cost of multifrequency optical sources, WDM for fiber distribution networks as they have heretofore been envisioned is not deemed cost-competitive with conventional power splitting passive optical networks or even simple point-to-point schemes (one fiber per customer). Accordingly, there is a continuing need for an efficient and cost-effective WDM system that is capable of transmitting a large number of spectral channels.